The present invention relates generally to a magnetoresistive sensor for use in a magnetic read head. More particularly, the present invention relates to current-perpendicular-to-plane (CPP) magnetoresistive (MR) read sensors having a reduced operating temperature.
Magnetoresistive read sensors, such as giant magnetoresistive (GMR) read sensors, are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. Depending on the geometry of the device, the sense current may be passed in the plane (CIP) of the layers of the device or perpendicular to the plane (CPP) of the layers of the device. The resulting signal can be used to recover the encoded information from the magnetic medium.
A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer, or between two ferromagnetic free layers. In the former case, the magnetization of the SAF is fixed, typically normal to an air bearing surface (ABS) of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The SAF includes a reference layer and a pinned layer which are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. In the latter case, the magnetizations of the two free layers rotate freely in response to an external magnetic field. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF, or as a function of an angle formed between the magnetization directions of the two free layers. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
A TMR read sensor is similar in structure to a CPP GMR spin valve, but the physics of the device are different. For a TMR read sensor, rather than using a spacer layer, a barrier layer is positioned between the free layer and the SAF or between two free layers. Electrons must tunnel through the barrier layer. A sense current flowing perpendicularly to the plane of the layers of the TMR read sensor experiences a resistance which is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF, or between the two free layers.
A pinning layer is typically exchange coupled to the pinned layer of the SAF to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atomic planes are aligned in alternating directions and, thus, there is no net magnetic moment in the material.
An underlayer is typically used to promote the texture of the pinning layer consequently grown on top of it. The underlayer is typically formed of a ferromagnetic material and is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction.
A seed layer is typically used to enhance the grain growth of the underlayer consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size.
As magnetic storage and retrieval systems have developed greater capacities, greater areal bit densities have been employed. Smaller areal bit sizes have corresponded to the greater areal bit densities. Narrow reader widths are desired for retrieval of data stored on ultra-high density media having small areal size bits. As the width of the read gap decreases with increasing areal bit densities, higher demand is placed on the sensitivity of the magnetoresistive portion of the MR read sensor. One effect of a decreased read gap width is an increase in the amount of heat that is produced by the magnetoresistive element during operation. When a MR read sensor is subjected to increasing temperatures, the thin layers comprising the MR read sensor become less reliable and more susceptible to thermal breakdown due to electrical and thermal stresses. The degradation of the function of the MR read sensor is exponentially dependent on the operating temperature of the device.
Another issue facing MR read sensors are temperature spikes resulting from electrostatic discharge (ESD) or electric overstress (EOS) transient and thermal asperity events during operation. Magnetic heads possess an acute sensitivity to damage or failure from ESD or EOS events.
Maintaining a low operating temperature is essential for the continued proper operation of MR read sensors and for proper read/write head electrical performance. The present invention is directed to maintaining such low operating temperatures in CPP MR read sensors.